Many parts of pressure resistant part materials of containers and piping, which are used in maximum temperature portions of subcritical pressure boilers or supercritical pressure boilers in power generating plants, and waste heat boilers in combined cycle power plants, or in semi-high temperature portions of ultra-supercritical pressure boilers in such plants, are composed of carbon steel or low alloy steel. To enhance the corrosion resistance of a site of a base material, such as carbon steel or low alloy steel, which contacts water or steam, it is common practice to build-up weld or overlay a weld metal, such as stainless steel or Ni-base alloy, on the surface of the base material, thereby covering the surface of the base material with the weld metal.
If the base material is fused during welding, the base material and the weld metal mingle, whereby the amount of Cr or Ni added to the weld metal is decreased. Such dilution needs to be suppressed maximally. Welding methods, which suppress the above-mentioned dilution of the weld metal, include a clad welding method which performs welding while weaving a welding torch in a direction perpendicular to the direction of welding (see, for example, Patent Document 1).
This clad welding method will be described by reference to FIG. 6 which is a drawing for illustrating the clad welding method according to a conventional technology. As shown in this drawing, a plasma are 103 is generated between a plasma torch 101 and a base material 102, whereby a molten pool 104 of a predetermined size is formed in the base material 102. A welding wire 105 as a weld metal is fed to this molten pool 104, whereas the plasma torch 101 is moved in a direction I, where welding proceeds, while being woven in a predetermined cycle. By so doing, a weld bead 106 comprising a part of the welding wire 105 melting into the base material 102 is formed. In FIG. 6, the numeral II denotes the locus of the plasma torch 101, and the numeral III denotes a direction in which the welding wire 105 is fed.
As described above, the welding wire 105 is fed to the molten pool 104, whereas the plasma torch 101 is moved in the welding proceeding direction I while being woven. By this measure, the weld bead 106 with minimal penetration of a weld metal 107 into the base material 102 is formed, as shown in FIG. 7(a). In the drawing, the numeral 108 denotes a penetration region where the weld metal 107 melts and penetrates into the base material 102.
In performing clad welding over a wide range of the base material 102, it is necessary to form a plurality of weld beads, and superpose one end of a preceding weld bead which precedes, and other end of a succeeding weld bead which succeeds. Assume here that welding is performed, with one end 106a of the preceding weld bead 106 being superposed by other end 116b of a succeeding weld bead 116, as shown in FIG. 7(b). In this case, at the other end 116b of the succeeding weld bead 116, a weld metal 117 does not fully melt into the base material 102, and lack of fusion or incomplete fusion tends to occur. In FIG. 7(b), the numeral 117 denotes the weld metal in the succeeding weld bead 116, and the numeral 118 denotes a penetration region where the weld metal 117 in the succeeding weld bead 116 melts into the base material 102. Thus, as shown in FIG. 7(c), a weld metal 109 in the one end 106a of the preceding weld bead 106 is removed by a grinder, and welding is performed, with the other end of the succeeding weld bead being superposed on this removed site 109. By so doing, the occurrence of the above-mentioned incomplete fusion has been prevented.